III nitride semiconductor devices such as field effect transistors (FETs), light emitting diodes (LEDs), which each have an element part formed from a Group III nitride semiconductor using Al, Ga, In, or the like as a Group III element and using N as a Group V element, are studied.
A III nitride semiconductor such as GaN or AlGaN typically has a hexagonal wurtzite crystal structure. In cases where a III nitride semiconductor such as GaN is epitaxially grown on a substrate of a different type, such as sapphire, typically, a layer is grown in the c axis direction, and the surface of the layer to be grown is the (0001) plane, which is called the Ga-face, whereas the surface on the opposite side is the (000-1) plane (the side in contact with the substrate). Further, for a III nitride semiconductor substrate such as a GaN substrate, typically, a surface on one side is the (0001) plane, whereas the surface on the opposite side is the (000-1) plane.
Here, JP 2004-071657 A (PTL 1) describes a technique of forming an active element portion composed of a III nitride semiconductor layer on the (0001) plane of an n-type GaN substrate; forming, in a III nitride semiconductor device provided with a p electrode on the active element and an n electrode on the (000-1) plane side of the n-type GaN substrate, pyramidal projections having a certain facet on the surface by wet etching or the like on the (000-1) plane side of the n-type GaN substrate; and forming an n electrode of Ti/Al or Ti/Au so as to cover the protrusions, thus obtaining a good ohmic contact.
GaN substrates and SiC substrates are still expensive, and conductive single crystal substrates having a large diameter are not available at low cost. Thus, a semiconductor is commonly grown on a sapphire substrate.
However, a sapphire substrate is electrically insulating; accordingly, electric current does not flow in the substrate. Therefore, in conventional devices, a semiconductor laminate formed by sequentially growing an n-type III nitride semiconductor layer, an active layer, and a p-type III nitride semiconductor layer on a sapphire substrate is partially removed to expose the n-type III nitride semiconductor layer. Further, a lateral structure has been employed in which an electric current is flown in a lateral direction by providing an n-side electrode and a p-side electrode on the exposed n-type III nitride semiconductor layer and the p-type III nitride semiconductor layer, respectively.
In contrast, in recent years, techniques of obtaining devices having a vertical structure as follows are studied. First, a buffer layer is formed on a sapphire substrate, which buffer layer is to be removed by, for example, laser irradiation, etching, or the like, and a semiconductor laminate is then formed, which laminate includes an n-type III nitride semiconductor layer, an active layer, and a p-type III nitride semiconductor layer. Next, after forming a conductive support on the semiconductor laminate, which support supports the semiconductor laminate, the buffer layer is decomposed by laser irradiation or selectively dissolved by etching, thereby separating (lifting off) the sapphire substrate. The support and the semiconductor laminate are sandwiched between a pair of electrodes to form a device. Note that a “buffer layer” here is a buffer layer for epitaxial growth of a semiconductor laminate, which buffer layer also serves as a lift-off layer for separating the semiconductor laminate from a sapphire substrate. The methods of fabricating III nitride semiconductor devices as described above are referred to as laser lift-off method and chemical lift-off method.